Often times players will be told well in advance of the upcoming patch a nerf is taking place. Primarily to keep players informed on modified game mechanics. And also to generate discussion.
A Stealth Nerf occurs when a game company does not announce a particular change to a game in the patch. Often times the change will simply be noted in the Patch notes, buried under other layers of changes players do not often read. Leading to many players confusion as to the change.
It also serves to eliminate resistance to the change the producer feels is necessary, as often preventing a Nerf is much easier then reversing a Nerf.